Strong reproductive isolation affects sexually selected traits via shared mechanistic links

For sexually reproducing organisms, much of their fitness is a product of how well they compete against rivals for mating opportunities. This "sexual selection" was identified as a potentially strong driver of evolution by Darwin, and has since been invoked to describe broad scale biodiversity. For sexual selection to contribute to species diversity, there must be a direct mechanistic connection between sexual selection and the traits that reduce mating/gene flow between different species, resulting in reproductive isolation and speciation. However while sexual selection is commonly asserted to be a driver of speciation, the evidence for a connection between sexual selection and reproductive isolation is almost entirely indirect. In my dissertation I provide evidence for direct genetic connections between these two processes and to assess the consequences of these shared genetic mechanisms for sexual selection and reproductive isolation. I first demonstrate a link between sexual selection and reproductive isolation confirming that sexual interactions may lead to rapid evolution of mate discrimination between populations of Caenorhabditis remanei. Second, in a complementary project in Drosophila melanogaster, I demonstrate that genes underlying a trait known to be important to sexual selection (sperm competition within species) also contribute to a trait important for reproductive isolation (sperm precedence between species). Third, using a theoretical model and simulations, I show that strong selection for sperm precedence can deplete genetic variation for sperm competition genes, indicating that selection for reproductive isolation can constrain responses to sexual selection. Lastly, I evaluate patterns of conspecific sperm precedence and sperm competition in natural populations of Drosophila pseudoobscura that vary in whether they co-occur with closely related species (which imposes stronger selection for reproductive isolation) and support the hypothesis that strong selection for increased reproductive isolation has collateral effects on intraspecies sexual selection.